1. Field of the Invention
The present invention relates to a procedure for evaluating the friction characteristics of a shouldered threaded connection or rotary threaded connection.
More specifically, the present invention relates to a procedure for evaluating, directly on the field, the friction characteristics of a general rotary threaded connection in a drillstring of a well for the production of hydrocarbons.
2. Discussion of Background
As it is known, drillstrings used in the drilling of the ground consist of a series of drill pipes joined to each other by rotary threaded connections. This type of connection guarantees, by pin/box contact, typically on a shoulder, the sealing of the mud inside the drillstring with respect to that outside.
The operating limits of a general shouldered threaded connection, and in particular those used in drilling operations, strictly depend on the make-up torque values applied to the drill pipe to be joined before putting them into action in the well. The determination of the optimum make-up torque value, for each pipe of the drillstring, is therefore an important condition for enabling the drilling operation with extremely precise safety margins and reliability.
The known methods presently used for determining the optimum make-up torque value are mainly based on a certain formula in whose determination the total K.sub.a friction parameter plays a determinant role; the latter puts the torque value during the make-up M.sub.up in relation with the forces which develop both on the shoulder and along the thread, Q.sub.up, according to the relation: EQU M.sub.up =K.sub.a Q.sub.up
The total K.sub.a friction parameter, sum of those relating to the shoulder, K.sub.s, and along the thread, K.sub.t, depends on several factors such as the geometry of the connection, the compound placed between the threads of the connection and the conditions of the contact surfaces (shoulder and thread) and may therefore vary within wide ranges during the duration of the connection depending on the variation of the above factors.
With this situation, the optimum make-up torque value can be obtained if the connection can be energized by producing a certain Q.sub.up value therein which is considered optimum. Consequently when the total K.sub.a friction value varies the optimum make-up value to be given to the connection, also varies.
An old API (American Petroleum Institute) regulation suggested that the optimum make-up value be measured independently of the compound placed between the threads which instead, as already mentioned, also determines the basic variations of the total friction parameter and therefore the optimum make-up.
API subsequently corrected this error by introducing the concept of the relative R.sub.f friction factor with which K.sub.a, in the presence of a certain lubricating compound can be corrected on the basis of a reference K.sub.a,ref value, corresponding to the application on the thread of a reference compound, thus satisfying the following equation: EQU K.sub.a =R.sub.f K.sub.a,ref
In particular, the new API regulation suggests a procedure for evaluating the relative R.sub.f friction factor which is basically based on the comparison between the slope of the torque/turn diagrams obtained by tightening, in the laboratory, a bolt previously treated with a reference lubricating compound and then with the compound to be examined.
FIG. 1, which is examplative, shows an example of a torque/turn diagram suitable for this type of calculation. In the ordinate there is the torque (M) applied during the make-up, in the abscissa the value of the relative rotation angle (.OMEGA.) between the pin and the box of the connection.
This angle starts being measured as soon as there is contact on the shoulder of the connection.
From the make-up of the bolt two curves are obtained having a different slope, tg.o slashed..sub.up relating to the make-up of the bolt treated with compound being tested and tg.o slashed..sub.up,ref with the reference compound. The relative friction factor results from the following equation : EQU R.sub.f =tg.o slashed..sub.up /tg.o slashed..sub.up,ref
The system proposed by the known art to evaluate the total friction parameter has the disadvantage of not taking into consideration the actual connection and in particular the geometrical factor and the factor relating to the conditions of the contact surface of the connection which, as already mentioned, influence K.sub.a and therefore the optimum make-up torque value. In other words, the system of the known art indirectly evaluates the K.sub.a of the threaded connection at the beginning of its life-cycle and is not capable of taking into consideration the degree of wear of its contact surfaces.